The current technique of providing long, trackbound vehicles with wheel arrangements is generally based on utilizing bogies, that is, rotatable, load-carrying frame structures which comprise two or more wheel sets, wheel axles and wheel suspension and wherein at least two bogies support a car body, the concept car body in this context being defined as the device which is intended to contain and/or constitute the base for the useful load (or payload). A variety of solutions for steering such bogies through track curves, and devices for minimizing oscillating movements during running, have been described over the years and at present constitute examples of well-tried technique. Of necessity, however, this type of bogie is heavy.
The reason for using bogies is also explained by the fact that the frame in the bogie permits two suspension levels between track and car body as well as improved possibilities of suspension of brake and drive equipment.
There is a desire to make the vehicles simpler and lighter, both in view of purchase and maintenance costs and in view of energy consumption when starting and stopping the vehicles. This also gives rise to a desire to reduce the weight of the bogies. One way of achieving bogies with lower weight is to use single-axle bogies. The wheel axle in a single-axle frame must be given the possibility of steering through track curves, while at the same time it is desired to counteract tendencies towards oscillation and winding of the bogie when driving at higher speeds.
In track curves the wheel axles in a trackbound vehicle should be aligned radially or almost radially to the track curve to make it possible to negotiate curves without friction arising and without significant wear of wheels and rails. Steering a wheel axle through a track curve means that the wheel axle is aligned radially to the track curve, but may just as well be expressed in such a way that the wheels are aligned and roll in the direction of a tangent of the rails, or almost along with the direction of a tangent of the rails.
Within railway engineering, there has long been a desire to find a practicable model of a single-axle bogie. It is known to try to equip trackbound vehicles with single-axle bogies with a wheel set, where the wheel set is journalled in a bogie frame and this bogie frame is rotatable around the vertical axis. In German patent specification DE 37 18 254, C2, referred to here as Di, a proposal for such a single-axle bogie is described. This specification describes a two-wheel bogie, the wheels of which are journalled in a bogie frame which is attached to the car body of the vehicle for taking up tractive efforts and braking forces. Further, the direction of rolling of the wheels is determined by a guiding device. In addition, via resilient elements the wheels are movable vertically and in lateral direction in relation to the car body and tangentially alignable in relation to the direction of track. To make possible radial guiding of the bogie through a track curve, the bogie is adapted to be rotatable around the vertical axis. A guiding device which consists of, for example, guide rods extending from an adjacent bogie of the train transmits steering forces to the bogie wheels such that the wheels are aligned tangentially to the track direction in a track curve. In addition, the bogie frame is connected to the car body through at least one link arm which extends in the longitudinal direction of the vehicle and which allows the rotation of the bogie frame around the vertical axis, but which at the same time prevents the bogie from tilting over around the transverse axle of the bogie during braking and acceleration.
The tread of a railway wheel is usually slightly conically shaped. The wheel has a certain conicity. According to a well-known pattern, this means that a wheel set or a bogie rotatable around the vertical axis displays a tendency to describe a sinusoidal motion when travelling on straight track. This property contributes to reduce the critical speed when driving a trackbound vehicle. The guiding device according to patent specification D1, mentioned above, also has the task of counteracting the oscillating and winding tendencies of the single-axle bogie when travelling on straight track.
A considerable disadvantage of the solution that the introduction of a single-axle bogie described in patent specification D1 entails is the need of the long guide rods which transmit the steering forces to the bogie. These guide rods are clumsy and unwieldy, especially when extending from the front to the rear bogie in the same vehicle, where the guide rods may have a length of over 10 m. In addition, under certain conditions, incorrect steering forces are transmitted via these guide rods, for example when a vehicle enters and exits a track curve and when passing through points with two adjacent and consecutive curves in different directions, where the radial alignment of a wheel axle to the track direction is to be different from that which the guide rods of the guiding device tend to align. This may cause extra wear on wheels and rails and in critical situations an increased risk of derailment. In addition, the guide rods described can cause maintenance problems, such as the need for accurate alignment and problems with wear and play in the joints of the guide rods. Furthermore, the long guide rods may cause oscillations and vibrations in the vehicle.
From U.S. Pat. No. 4,067,261 it is known to design a bogie with self-steering, that is, that the wheels in the bogie are aligned tangentially to the rails in a track curve, by the treads of the wheels having elevated conicity and hence strive to align the bogie in the track direction. However, the device according to the this patent only relates to two- or multi-axle bogies or single wheel axles attached directly to a car body. This patent does not describe any solution regarding self-steering single-axle bogies or demonstrate any necessary auxiliary devices for achieving a well-functioning self-steering single-axle bogie.
The conicity of the tread of a railway wheel is a measure of the degree of increasing wheel radius measured in a radial section across the tread of the wheel in a direction towards the wheel flange. Usually, the concept effective conicity is used, since the wear on the tread of a wheel changes the original purely conical shape of the tread of the wheel, whereby the tread may assume a saddle shape. By effective conicity is then meant the ratio of the change of the radius of rolling of the wheel to the lateral movement of the wheel across the rail.